Cadmium plating compositions



United States Patent 3,014,852 CADMIUM PLATENG COMPOSITIONS William F.Hamilton, Altadena, and Welton R. Burney, In, Northridge, Calilh,assignors to Lockheed Aircraft Corporation, Burbank, Calif. No Drawing.Filed Aug. 26, 1957, Ser. No. 680,397 11 Claims. (Cl. 204-50) Thisinvention relates to the electrodeposition of cadmium and relates moreparticularly to electroplating baths useful in the electroplating ofcadmium on ferrous metals. A general and important object of theinvention is to provide for the effective dense deposition of cadmiumfrom aqueous alkaline cadmicyanides without hydrogen embrittlement ofthe articles being plated.

The copending application of William F. Hamilton et al., Serial Number568,944, filed February 29, 1956, discloses aqueous cadmicyanide platingcompositions for the non-embrittling cadmium plating of ferrous metals,and the like, which utilize, essentially, a high percentage of cadmiummetal and a low percentage of free cyanide with nitrate ions included toprevent the formation of hydrogen at the cathode under ordinaryoperating conditions and thus effectively avoid embrittlement of theparts being plated. The high percentage of cadmium metal, the lowpercentage of free cyanide and the nitrate ions are all required fornon-hydrogen embrittlement over the normal ranges of current densitiesencountered in cadmium plating bath operations. In the plating bathcompositions of the copending application small amounts of solublealkylamines are utilized to increase both the throwing power and theplating efiiciency.

An important object of the present invention is to improve the throwingpower of non-embrittling aqueous cadmium cyanide electroplatingsolutions of the general character disclosed in the aforementionedcopending application. It has been found that the amines when employedas throwing power additives tend to evaporate from the solutions andsince the relative proportion of the additive must be carefully andaccurately maintained such evaporation may cause a poor distribution ofthe plate and a reduced overall plating efficiency. On the other hand,any substantial excess in the amount of the amine additive will causethe plated parts to become embrittled at the current densities normallyemployed in the bath. We have discovered that in such a solution of highcadmium metal content and low free cyanide content, containing nitrateions, the addition of Water soluble polyethylene glycols materiallyincreases both the efficiency of the plating operation and the throwingpower of the solution and the problem of appreciable evaporation of theadditive is avoided. Thus when a current density of 72 amperes persquare foot is employed the cathode efficiency (with regard to thedeposition of the cadmium), of a typical non-embrittling platingsolution of the character hereinabove referred to, is less than 20%whereas with the addition to the solution of only approximately 0.05% byweight of mixed polyethylene glycols this efliciency increases to over80%.

The non-hydrogen embrittling aqueous cadmium cyanide electroplatingsolutions of this invention may be said to comprise, generally, arelatively high percentage of cadmium metal, a relatively low percentageof free cyanide, hydroxide, nitrate ions for preventing or reducing theformation of hydrogen at the cathode and a relatively small proportionof water soluble polyethylene glycols for increasing both the throwingpower and the plating efiiciency of the bath.

While the electroplating compositions of this invention are useful inthe cadmium plating of various metals we will herein refer to theplating of ferrous alloys having high ultimate tensile strengths, forexample in the 3,014,852 Patented Dec. 26, 1961 range of from 260,000 to280,000 pounds per square inch. Such steels or alloys are frequentlyemployed in the fabrication of aircraft parts and elements such aslanding gear, fuel tank attachments, motor mounts, and the like. Inpractice the weight saving effected by the employment of such highstrength ferrous alloys in the construction of an airplane, may amountto several hundred pounds in a given large airframe. The susceptibilityof these high strength ferrous alloys to rusting is a markeddisadvantage and wherever possible the parts or members constructed ofsuch alloys are cadmium plated. The cadmium plating in addition tocovering the exposed surfaces of the ferrous alloy members providesanodic protection against corrosion should the plating become marred orscratched. The cadmium plating of the steel in conventional cyanidesolutions is usually attended by the evolution of hydrogen and thehydrogen acts to embrittle the steel. Detection of such hydrogenembrittlement cannot be effected without destruction of the part orarticle tested and it is, therefore, impossible to properly demonstratewhether embrittlement has adversely affected a given part prior to itsactual incorporation in the airplane. Since hydrogen embrittlement isquite common and its detection is difficult, it is not uncommon for apart or member of high strength steelto unpredictably fail in servicedue to hydrogen embrittlement occasioned by the cadmium platingoperations. The effective cadmium plating of steel parts Withouthydrogen embrittlement thereof is accord-' ingly extremely important,especially where failure of an embrittled member or part may cause afatal accident or disaster. The present invention is, therefore,directed to the electroplating of cadmium by means of solutions orcompositions which. avoid hydrogen embrittlement of the ferrous metalparts and yet obtain continuous adherent dense coatings of the cadmiummetal regardless of the shape or contour of the parts plated. As aboveindicated, this is effected by incorporating small amounts ofpolyethylene glycol or mixed polyethylene glycols in solutions of highcadmium metal content, low free cyanide content and containing nitrateions.

Another and important object of the invention is to providecadmiumplating solutions of the character mentioned having ample andeffective throwing power to assure adequate deposition of the cadmiummetal in crevices, in the roots of threads, and on other irregularitiesand surfaces that are normally difiicult to plate and that mightnormally cause rough, discontinuous or irregular plating. A cadmiumelectroplating solution of high cadmium, low free cyanide content andcontaining nitrate ions normally has poor throwing power, and lowplating efficiency. We have discovered, however, that the incorporationof mixed polyethylene glycolsin such a solution imparts good throwingpower and high or effective plating efficiency to the solution to assurethe deposition of the cadmium in the roots of threads and on othersurfaces otherwisedifricult to properly plate.

Under extremely good plating conditions a smooth symmetrical part devoidof corners, crevices, etc.'may be effectively cadmium plated with thesolutions of the invention without any hydrogen embrittlement andwithout incorporating polyethylene glycol. However, in actual practicein plating operations where the parts to be plated have threads, abruptcorners, or other surfaces diflicult to plate, the current densities ofthe plating bath are, of necessity, inherently varied in the bathbecause of the function of the electrical resistance thereinf We havediscovered that by incorporating relatively small amounts of mixedpolyethylene glycols the throwing power of the bath or solution isincreased and automatically compensates for the necessarily variedcurrent densities in the bath at the surfaces of the part being plated.Thus the cadmium plating solutions of this invention incorporating thesmall proportion of the mixed polyethylene glycols have sufiicient andefiective throwing power and are adequately efiicient to assure theproper deposition of. the cadmium metal on the parts, irrespective oftheir shapes. It is, therefore, an objective to provide cadmium platingsolutions of the character described having relatively small amounts ofan additive in the nature of mixtures of glycols of difierent averagemolecular weights, that isditferent degrees of polymerization, to assureproper effective plating at the various current densities that-are ofnecessity present in cadmium plating baths.

Another object of our invention is to provide electroplating solutionsof this class composed of readily available materials compatible withthe ingredients now commonly employed in the cadmium plating art andwhich enable the plating operations to be successfully and expeditiouslycarried out without hydrogen embrittlement of the parts being plated.

In employing the present invention cadmium metal is plated from anaqueous cyanide solution containing a relatively high percentage ofcadmium metal and a relatively low percentage of free cyanide withnitrate ions added to prevent the formation of hydrogen at the cathodeunder the ordinary plating conditions and thus effectively avoidingembrittlement of the parts being plated. Conventionally, the addition ofeven small amounts of nitrate ions to cadmicyanide plating baths isavoided since both the efiiciency of the bath and the quality of theresultant plating are adversely affected thereby. In employing theplating solutions of the present invention however the decrease of theefiiciency of the bath due to the nitrate is counteracted by theadditive. Another commonly recognized disadvantage of a high cadmium,low free cyanide plating bath containing added nitrate is the markedreduction in throwing power which means that the solution will usuallyor often fail to properly deposit the cadmium metal in the roots ofscrew threads, in narrow crevices, and on other irregularities of theplated parts, and the sensitivity of the plating to the condition of thesurfaces of the article is extremely critical. Thus slight films ofsurface oxide, almost imperceptible grease films, and the like, may makethe deposited cadmium plating rough, irregular, or discontinuous. Inaccordance with the present invention this disadvantage of the highcadmium, low free cyanide baths containing the added nitrate is alsoavoided and practically eliminated by the inclusion and maintenance inthe bath of a small amount of mixed water soluble polyethylene glycols.The polyethylene glycols increase both the throwing power and theefficiency of the solution, reduce the sensitivity of the plating tosurface conditions of the plated part and impart improved platingcharacteristics to the bath or solution. Under typical operatingconditions we have found that the inclusion of the polyethylene glycolshaving selectedaverage molecular weights enables the production ofexcellent fine-grained cadmium plate from suitably formulated solutionsat current densities varying from less than 20 to more than 100 amperesper square foot. Amperes per square foot is herein abbreviated a.s.f.

The degree of polymerization of the ethylene glycols utilized in theplating baths has a pronounced effect on the cadmium deposition atdifferent current densities. In general polyethylene glycols havingaverage molecular weights up to approximately 500 are most effective inproducing improved plate at current densities exceeding about 100 a.s.f.while polyethylene glycols having average molecular weights in the rangeof from about 500 to about 1000 are most effective in the intermediateden- 7 sity ranges of from 50 to 100 a.s.f. and where the averagemolecular weights of the polyethylene glycols are betweenapproximately1000 and 1500 the most pronounced elfect in increased throwing power andplating elliciency v is. with thelower current plating density ranges,for example upto approximately 50 a.s.f. g For good results under normalplating conditions with the high cadmium metal, low free cyanide andnitrate bath, approximately 0.066 ounce per gallon of mixed polyethyleneglycols may be employed. In such a case the polyethylene glycols maycomprise a mixture of about 40% by weight having average molecularweights up to 500, about 40% by weight having average molecular weightsbetween 500 and 1000 and about 20% by Weight polyethylene glycols havingaverage molecular weights between 1000 and 1500. We have foundthat whena total of less than 0.027 ounce per gallon of such a mixture of thesoluble polyethylene 'glycols is used, the bath has a lowered platingeiiiciency and a reduced throwing or covering power. Where theproportion of the mixed polyethylene glycols is appreciably above 0.066ounce per gallon the increase in plating efiiciency and throwing orcovering power is relatively small and unimportant. If a totalconcentration of the mixed polyethylene glycols is over about 0.13 ounceper gallon a slight permanent precipitate may appear in the bath sincethe limit of solubility of some of the higher molecular weightpolyglycols may have been exceeded. Such a precipitate should be removedby filtration to avoid the formation of spots or stains on the cadmiumplate.

The relative proportions of the several ingredients malt ing up theimproved electroplating baths of this invention are, of course, subjectto considerable variation without bringing about hydrogen embrittlementof the ferrous metal parts being plated while employing normal ranges ofcurrent densities as conventionally utilized in such plating operations.We have found it is most desirable to maintain the nitrate content ofthe baths expressed in terms of sodium nitrate between approximately 3and 28 ounces per gallon of the plating solution. The total cadmiummetal content of the plating solution should preferably be betweenapproximately 14 and 18 ounces per gallon. It has been noted that if thecadmium metal content is excessively low, hydrogen embrittlement of theplated metal may be encountered while if the cadmium metal content isexcessively high, precipitation may result. While we have above referredto the nitrate content of the bath being expressed in the terms ofsodium nitrate it is to be understood that nitrate ions may beincorporated in the, solutions in other manners. For example, nitrateions may also be added to the bath in the form of cadmium nitrate, thussimultaneously increasing the cadmium metal content of the solution. Thenitrate ions may be added in Whole or in part as sodium nitrate. In theplating solutions the nitrate ion is in a very alkaline medium and is.normally surrounded by a stoichiometric excess of sodium ions so that itmay be considered as existing as sodium nitrate in the solution for allpractical purposes. Thus it will be seen that the addition of nitrateions as sodium nitrate has substantially the same effect as introducingthe ions in other forms.

Itv has been found that where the bath or plating solution containsabout 16 ounces per gallon cadmium metal the total cyanide content interms of sodium cyanide should be between 20 andv 25 ounces per gallonand preferably about 22-23. The free sodium cyanide present in theplating solution determines the rate of increase of cadmium metal andthe total cyanide present governs the total amount of the metal in thesolution. In the event the free sodium cyanide exceeds about 4.00 ouncesper gallon of the solution hydrogen embrittlement of the ferrous metalbeing plated may be induced due to decreased efiiciency of the bathwhereas if the free cyanide content of the solution falls below; say,0.7 ounce per gallon the quality of the plating may be adverselyaliected.

- As the plating operation progresses the sodium nitrate is. graduallyreduced to sodium nitrite and in order to prevent hydrogen embrittlementof the ferrous metal parts being plated due to local depletion of thenitrate constant agitation of the bath is important. It is pre ferred tomaintain the sodium hydroxide content of the plating bath betweenapproximately 5 and 10 ounces per gallon. However the concentration ofthe sodium hydroxide is not believed to be critical. The percentage orconcentration of the carbonate present in the solution is alsorelatively unimportant but in the event it builds up to an excessivevalue it may be removed by chilling and filtering the solution or byprecipitation as an insoluble carbonate.

The following are typical preferred formulations for the electroplatingbaths or solutions of the invention from which it will be seen thatsubstantial variation may be made without departing from the spirit ofthe invention.

Example 1 Prepare a solution as follows:

Ounces per gallon Sodium cyanide 27.3 Cadmium oxid 15.2 Sodium hydroxide2.7 Sodium nitrate 9.3 Mixed polyethylene glycols 0.054

' remain within the following approximate limits by analysis:

- Ounces per gallon Total sodium cyanide 20 to 25 Free sodium cyanide0.7 to 4 Cadmium 14 to 18 Sodium nitrate 3 to 28 Sodium hydroxide 5 toSodium carbonate A trace to 8 As the cadmium plating operation proceedsthe content or proportion of the mixed polyethylene glycols should becontrolled by making comparative tests in a Hull cell and preferably inno event should exceed a total concentration of about 0.1% by weight(0.13 ounce per gallon) or fall below a total concentration of 0.02% byweight (0.026 ounce per gallon).

Example 2 A cadmium plating solution containing about 10% cadmium metalin sodium cyanide may-be prepared as follows:

Ounces Sodium cyanide 22 Cadmium oxide In preparing the bath of Example2 the sodium cyanide is dissolved in about one-half gallon of distilledor deionized water and the cadmium oxide is then slowly added to thesolution which is stirr d continuously until the cadmium oxide iscompletely dissolved. 7.8 ounces of potassium nitrate is then added tothe solution .and water is added to bring the volume to approximatelyone gallon. The throwing power and etficiency increasing additive isthen incorporated by adding two milliliters of polyethylene glycolhaving an average molecular weight of approximately 400 and onemilliliter of polyethylene glycol with an average molecular weight ofapproximately 1000.

Example 3 With the addition of 0.03 ounce of polyethylene glycol havingan average molecular weight of about 600 this solution will platecadmium metal satisfactorily under ordinary plating conditions and withthe current density of 100 a.s.f. or more without noticeable hydrogenembrittlement.

Increase the final volume to 5.25 gallons by adding distilled orde-ionized water.

This solution of Example 4 will plate cadmium metal on steel withoutvisible cathodic gassing or without em brittlement with a currentdensity from as low as'about 20 a.s.f. to as high as about 100 a.s.f.Note that Example 4 contains a polyethylene glycol additive of a givenor relatively narrow molecular weight range. A bath or solutioncontaining such an additive may be used successfully where the parts tobe plated are symmetrical or relatively symmetrical so that the inherentelectrical resistances present in the bath are generally uniform.

It is of importance that the surfaces of the articles to be p.ated areproperly prepared. All grease, oil, and surface films should be removedbefore the articles are immersed in the plating bath. The plating bathor solution should be continuously agitated and the cathodic articleskept in motion to prevent local depletion of the bath or changes in theconcentration of its components. High strength steel parts or articlesmay be eifectively 1 plated'in the baths of our invention without thevisible evolution of gas at the cathode and without noticeable hydrogenembrittiement.

The baths or solutions may be maintained in operative condition by theusual methods well known to those skilled in the art. An excessiveincrease or build up of sodium carbonate may be reduced by chilling thesolution t. about 50 F. and decanting or filtering. Alternately asolution of barium nitrate may be added, for: example, to decrease thecarbonate by precipitating it as barium carbonate and simultaneouslyincreasing the sodium'nitrate content of the solution by doubledecomposition. As above pointed out, nitrate ions may be added to thebath in the form of cadmium nitrate thus simultaneously increasing thecadmium metal content of the solution. In the event the platingsolutions contain an excessive amount of cadmium hydroxide nitrate ionsmay be added as nitric acid. This should be done very slowly and withrapid stirring and extreme caution as the reaction of the acid isviolent and the hydrogen cyanide is toxic. The resulting sodium nitrateis neutral and the amount of free alkali is thus decreased. Higherproportions or concentrations of nitrate, approaching its limit ofsolubility, in the baths tend to prevent burning of the plate at thehigher current densities and do not appear to cause any salient untowardeffects.

It is to be understood that our invention is not to be construed asbased upon or dependent upon any theories which we have expressed. Noris the invention to be re garded as limited to the express procedures ormaterials set forth, these details being given only by way ofillustration and to aid in clarifying the invention. We do not regardsuch specific details as essential to the invention except insofar asthey are expressed by way of limitation in the following claims whereinit is our intention to claim all novelty inherent in the invention asbroadly as permissible in view of the prior art.

We claim:

1. A non-embrittling cadmicyanide electroplating solution comprisingfrom approximately 14 to approximately 18 ounces per gallon cadmiummetal, from approximately 0.7 to approximately 4 ounces per gallon freesodium cyanide, from approximately 3 to approximately 28 ounces pergallon alkali metal nitrate, and from about 0.027 to about 0.13 ounceper gallon of water soluble mixed polyethylene glycols having dilferentaverage molecular weights ranging between approximately 300 and 1500.

2. A non-embrittling cadrnicyanide electroplating solution comprisingfrom approximately 14 to approximately 18 ounces per gallon cadmiummetal, from approximately 0.7 to approximately 4 ounces per gallon freesodium cyanide, from approximately 3 to approximately 28 ounces pergallon alkali metal nitrate, and from about 0.027 to about 0.13 ounceper gallon of water soluble mixed polyethylene glycols having differentaverage molecular weights ranging from 300 to 1500.

3. A non-embrittling cadmicyanide electroplating solution comprisingfrom approximately 14 to approximately 18 ounces per gallon cadmiummetal, from approximately 3 to approximately 28 ounces per gallon alkalimetal nitrate, and from 0.027 to about 0.13 ounce per gallon of watersoluble mixed polyethylene glycols having dilierent average molecularweights ranging between approximately 300 to 1500.

4. A non-embrittling cadmicyanide electroplating solution comprisingfrom approximately 14 to approximately 18 ounces per gallon cadmiummetal, from approximately 3 to approximately 28 ounces per gallon alkalimetal nitrate, and from 0.027 to 0.13 ounce per gallon of water solublemixed polyethylene glycols having different average molecular weightsranging from about 300 to about 1500.

5. An aqueous electroplating bathcontaining in terms of ounces pergallon, from 20 to 25 ounces sodium cyanide, from 14 to 18 ouncescadmium, from 3 to 28 ounces sodium nitrate, from to ounces sodiumhydroxide, and about 0.05 ounce mixed water solublepolyethylene glycolshaving difierent average molecular weights ranging from 300 to 1500.

6. An aqueous electroplating bath containing in terms of ounces pergallon, about 27 ounces sodium cyanide, about ounces cadmium oxide,about 3 ounces sodium hydroxide, about 9 ounces sodium nitrate, andabout 0.05 ounce polyethylene glycol having an average molecular weightranging between approximately 300 and 1500.

7. An aqueous electroplating bath containing in terms of ounces pergallon, about 27 ounces sodium cyanide, about 15' ounces cadmium oxide,about 3 ounces sodium hydroxide, about 9 ounces sodium nitrate, and from0.027 to about 0.13 ounce mixed polyethylene glycols having differentaverage molecular weights ranging from 300, to 1500.

8. An aqueous electroplating bath containing in terms of ounces pergallon, about 27 ounces sodium cyanide, about 15 ounces cadmium oxide,about 3 ounces sodium hydroxide, about 9 ounces sodium nitrate, and fromabout 0.027 to about 0.13 ounce of substantially equal parts by weightof mixed polyethylene glycols having different average molecular weightsof approximately 300, 6,00, 1000 and 1500 respectively.

9. An electroplating solution containing about 1 gallon water, about14.5 ouncescadmium nitrate tetrahydrate, about 26.6 ounces sodiumcyanide, about 17.7 ounces cadmium oxide, about 2.6- ounces sodiumhydroxide, and about 0.03 ounce polyethylene glycol having an averagemolecular weight of about 600.

10. An electroplating solution comprising approximately 5.25 gallonswater, 9 pounds sodium cyanide, 5 pounds cadmium oxide, 6.6 poundssodium nitrate, and 0.15 ounce polyethylene glycol having an averagemolecular weight of about 400. V

11. A non-embrittling cadmicyanide electroplating solution comprisingfrom approximately 14 to approximately 18 ounces per gallon cadmiummetal, from approximately 0.7 to approximately 4 ounces per gallon freesodium cyanide, from approximately 3 to approximately 28 ounces pergallon alkali metal nitrate, and from about 0.027 to about 0.13 ounceper gallon of water soluble mixed polyethylene glycols having differentaverage molecular weights ranging from 300 to 1500 comprising about 40%by weight polyethylene glyco-ls having an average molecular weight up to500, about 40% polyethylene glycol having average molecular weightsbetween 500 and 1000 and about 20% by weight polyethylene glycols havingaverage molecular weights between 1000 and 1500.

7 References Cited in the file of this patent UNITED STATES PATENTS,

2,107,806 Soderberg et al. Feb. 8, 1938 2,848,393 Foulke et a1 Aug. 19,1958 2,881,121 Foulke et al. Apr. 7, 1959 2,892,761 Hamilton et al. June30, 1959 FOREIGN PATENTS 464,545 Canada Apr. 18, 1950 OTHER REFERENCESFlora: American Journal of Science, vol. 20 (1905'), pp. 454-455.

1. A NON-EMBRITTLING CADMICYANIDE ELECTROPLATING SOLUTION COMPRISINGFROM APPROXIMATELY 14 TO APPROXIMATELY 18 OUNCES PER GALLON CADMIUMMETAL, FROM APPROXIMATELY 0.7 TO APPROXIMATELY 4 OUNCES PER GALLON FREESODIUM CYANIDE, FROM APPROXIMATELY 3 TO APPROXIMATELY 28 OUNCES PERGALLON ALKALI METAL NITRATE, AND FROM ABOUT 0.027 TO ABOUT 0.13 OUNCEPER GALLON OF WATER SOLUBLE MIXED POLYETHYLENE GLYCOLS HAVING DIFFERENTAVERAGE MOLEUCLAR WEIGHTS RANGING BETWEEN APPROXIMATELY 300 AND 1500.